This invention relates to an improved semiconductor apparatus, and more particularly, it relates to an improved, flat-base or stud-type semiconductor apparatus having a low inductance.
FIG. 1 illustrates the structure of a conventional semiconductor apparatus, which in this case is a stud-type diode. This stud-type diode has a studded diode support 1 which comprises a hexagonal base 1a which is made of copper, a generally cylindrical iron casing 1b, and a threaded stud 1c which is also made of copper. The base 1a and the threaded stud 1c are formed as a single member, while the casing 1b is secured to the upper surface of the base 1a by brazing. A circumferentially-extending flange 1d is formed on the outer surface of the casing 1b. The casing 1b houses a diode 2 which rests atop the base 1a and is in electrical contact therewith. The diode 2 supports a cathode rod 3 which has a circular base 3a and a rod portion 3b which extends perpendicularly upwards from the base 3a. The base 3a of the cathode rod 3 supports an electrically-insulating ceramic ring 4, which in turn supports a steel disk spring 5. The rod portion 3b of the cathode rod 3 passes through the holes in the centers of the ring 4 and the disk spring 5.
The casing 1b is covered by a cap 6 comprising an annular, electriclly-insulating, ceramic end wall 6a, a copper junction pipe 6b which is secured to the inside of the hole at the center of the end wall 6a, and a cylindrical lower portion 6c which is made of iron and which surrounds the casing 1b of the studded support 1. The ceramic end wall 6a is secured to the junction pipe 6b and the cylindrical lower portion 6c by brazing. the lower end of the cylindrical lower portion 6c is welded to the flange 1d of the casing 1b along its entire periphery so as to form an airtight seal. The rod portion 3b of the cathode rod 3 is electrically connected to a lead 7 by the junction pipe 6b. The lead 7 is made from copper stranded wire.
The conventional stud-type diode of FIG. 1 is assembled in the following manner. First, a diode 2 is inserted into the casing 1b of the studded support 1 and placed flat atop the base 1a. Next, the elecctrically-insulating ring 4 and the disk spring 5 are placed over the rod portion 3b of the cathode rod 3 with the ring 5 sandwiched between the upper surface of the base 3a and the disk spring 4. The cathode rod 3 is then inserted into the casing 1b, and the base 3a of the cathode rod 3 is placed atop the base 1a of the studded support 1. The diameter of the electrically-insulating ring 4 is substantially the same as the inner diameter of the casing 1b so that when the ring 4 is inserted into the casing 1b, the cathode rod 3 is automatically centered atop the diode 2.
Next, the disk spring 5 is compressed from above by a suitable pressing apparatus, and while the disk spring 5 is in a compressed state, the upper ends of the casing 1b are plastically deformed inwards, thereby preventing the disk spring 5 from returning to a relaxed state. As a result, the disk spring 5 exerts a downwards force on the ring 4 which presses the base 3a of the cathode rod 3 firmly against the diode 2 and the diode 2 firmly against the base 1a of the studded support 1.
The cap is then placed over the studded support 1 with the cathode rod 3 extending into the center of the junction pipe 6b. The lower end of the cylindrical lower portion 6c of the cap 6 and the flange 1a of the casing 1b are welded to one another to form an airtight seal. Lastly, a lead 7 is inserted into the upper end of the junction pipe 6b, and the junction pipe 6b is crimped by the application of external pressure so as to produce a good mechanical and electrical connection between the rod portion 3b, the walls of the junction pipe 6b, and the lead 7.
The above-described, conventional semiconductor apparatus has the drawbacks that the rod portion 3b of the cathode rod 3 is long and slender and is surrounded by a magnetic material in the form of the disk spring 5. It therefore has a high inductance when current is passing therethrough. Furthermore, because of the structure of the junction pipe 6b, the lead 7 must also be slender, which results in a high inductance. If such a semiconductor apparatus is used as part of a snubber for a gate turnoff thyristor, due to the high inductance of the apparatus, the spike voltage is large at turnoff time and the controllable current of the gate turnoff thyristor is low.